1. Field of the Invention
This invention relates to pulse density analyzers which provide an output indication when the pulse density input to the analyzer reaches a certain level and, more particularly, to a self-checking analyzer which provides a continuous self-check of the pulse density analysis.
2. Description of the Prior Art
Many prior-art error pulse density analyzers use a combination of a counter and a timer, such as is disclosed in U.S. patent application Ser. No. 583,147, filed June 2, 1975, entitled "Error Density Detector". When such error detectors receive an initial error pulse, a timer is started and error pulses are counted during the timing interval. A disadvantage of this prior-art technique arises when one tries to make the circuit self-checking. Basically, one of two methods of self-checking may be used. The first method consists of periodically sending a known-density error pulse train through the density calculator and verifying whether or not an alarm signal appears at the output. This method does not verify that the circuit is reacting at the correct density threshold because its counter could be overflowing earlier than it should. The second method of self-checking involves a redundant error density analyzer circuit. Since the timers use timing components with varying manufacturing tolerances, the two timers may have slightly different time-out times. This could initiate a self-check alarm since the two circuits have different outputs even though there is no circuit failure.